[PATCH v2 2/2] kernel/locking/rtmutex.c: some code optimization

[Thavatchai Makphaibulchoke]

Adding the following code optimization,

- Reducing the number of cmpxchgs.  Only call mark_rt_mutex_waiters() when
  needed, waiters bit is not set.
- Reducing the hold time of wait_lock lock.
- Calling fixup_rt_mutex_waiters() only when needed.
- When unlocking rt_spin_lock in IRQ, alternate between attempting fast
  unlocking and attempting to lock mutex's wait_lock.

Signed-off-by: T. Makphaibulchoke <t...@hp.com>
---
 kernel/locking/rtmutex.c | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index ae5c13f..cadba20 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -608,7 +608,8 @@ __try_to_take_rt_mutex(struct rt_mutex *lock, struct 
task_struct *task,
         * any more. This is fixed up when we take the ownership.
         * This is the transitional state explained at the top of this file.
         */
-       mark_rt_mutex_waiters(lock);
+       if (!((unsigned long)lock->owner & RT_MUTEX_HAS_WAITERS))
+               mark_rt_mutex_waiters(lock);
 
        if (rt_mutex_owner(lock))
                return 0;
@@ -832,8 +833,8 @@ static void remove_waiter(struct rt_mutex *lock,
        struct rt_mutex *next_lock = NULL;
        unsigned long flags;
 
-       raw_spin_lock_irqsave(&current->pi_lock, flags);
        rt_mutex_dequeue(lock, waiter);
+       raw_spin_lock_irqsave(&current->pi_lock, flags);
        current->pi_blocked_on = NULL;
        raw_spin_unlock_irqrestore(&current->pi_lock, flags);
 
@@ -1019,11 +1020,11 @@ static void  noinline __sched 
rt_spin_lock_slowlock(struct rt_mutex *lock)
                if (top_waiter != &waiter || adaptive_wait(lock, lock_owner))
                        schedule_rt_mutex(lock);
 
-               raw_spin_lock(&lock->wait_lock);
-
                pi_lock(&self->pi_lock);
                __set_current_state(TASK_UNINTERRUPTIBLE);
                pi_unlock(&self->pi_lock);
+
+               raw_spin_lock(&lock->wait_lock);
        }
 
        /*
@@ -1038,12 +1039,6 @@ static void  noinline __sched 
rt_spin_lock_slowlock(struct rt_mutex *lock)
        self->saved_state = TASK_RUNNING;
        pi_unlock(&self->pi_lock);
 
-       /*
-        * try_to_take_rt_mutex() sets the waiter bit
-        * unconditionally. We might have to fix that up:
-        */
-       fixup_rt_mutex_waiters(lock);
-
        BUG_ON(rt_mutex_has_waiters(lock) && &waiter == 
rt_mutex_top_waiter(lock));
        BUG_ON(!RB_EMPTY_NODE(&waiter.tree_entry));
 
@@ -1096,7 +1091,14 @@ static inline void rt_spin_lock_fastunlock_in_irq(struct 
rt_mutex *lock,
                return;
        }
        do {
+               /*
+                * Alternate between fast acquire and try lock and proceed
+                * to slow lock whichever succeeds first.
+                */
                ret = raw_spin_trylock(&lock->wait_lock);
+               if (!ret && unlikely(rt_mutex_cmpxchg(lock, intr_owner,
+                       NULL)))
+                       return;
        } while (!ret);
 
        slowfn(lock, intr_owner);
@@ -1499,10 +1501,12 @@ rt_mutex_slowtrylock(struct rt_mutex *lock, struct 
task_struct *task)
 
                ret = try_to_take_rt_mutex(lock, task, NULL);
                /*
-                * try_to_take_rt_mutex() sets the lock waiters
-                * bit unconditionally. Clean this up.
+                * try_to_take_rt_mutex() keeps the lock waiters
+                * bit set when failed to grab lock. Clean this
+                * in case of a failure.
                 */
-               fixup_rt_mutex_waiters(lock);
+               if (!ret)
+                       fixup_rt_mutex_waiters(lock);
        }
 
        raw_spin_unlock(&lock->wait_lock);
-- 
1.9.1

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